Activation of Skeletal Stem and Progenitor Cells for Bone Regeneration Is Driven by PDGFRβ Signaling

The characterization and comparison of femoral bone-derived skeletal stem cells

Skeletal stem cells (SSCs) reside in various niche locations within long bones to maintain bone homeostasis and facilitate fracture repair. Bone fragility, associated with ageing, increases the susceptibility of the femoral head to fractures due to an increase in bone adipocytes and concomitant loss of structural integrity. However, the specific contribution of epiphyseal SSCs to fragility is unknown. To explore this, a comparative analysis was performed on the transcriptional profiles and lineage commitment of Wistar rat femoral SSCs derived from the bone marrow (BM-), diaphyseal cortical bone (CB-) and proximal epiphyseal trabecular bone (PF-SSCs) isolated from the same long bones. SSCs were characterized based on morphology, immunophenotype (CD90/CD45), growth rate (population doubling time), gene expression profiles and differentiation capacity (Oil Red O, Alizarin Red S). qRT-PCR micro-arrays were performed on SSCs to evaluate the expression of stemness, SSC and lineage-specific markers in both undifferentiated and differentiated states. Our findings support the hypothesis that SSCs from different bone regions exhibit distinct transcriptional profiles, reflecting their specific niche environments. CB-SSCs displayed superior osteogenic potential as evidenced by the expression of key osteogenic genes and higher levels of mineralization. In contrast, PF-SSCs had a reduced osteogenic capacity with a higher adipogenic potential. Overall, the study revealed the importance of niche-specific stem cell properties for use in regenerative medicine applications and provides insight into the potential role of PF-SSCs in bone fragility and fracture risk.

Pdgfrβ marks distinct mesenchymal and pericyte populations within the periosteum with overlapping cellular features

Platelet-derived growth factor receptor β (Pdgfrβ) is a cell surface marker often present on mesenchymal progenitor cells, playing a key role in regulating cell proliferation, migration, and survival. In the skeleton, Pdgfrβ-positive cells have significant osteogenic potential, differentiating into osteoblasts after injury to promote bone repair and homeostasis. However, multiple cell types within bone tissue express Pdgfrβ and their overlapping or distinct cellular features remain incompletely understood. Using a combination of single-cell RNA sequencing and transgenic Pdgfrβ-CreERT2-mT/mG reporter mice, we examined Pdgfrβ+ cells in mouse long bone periosteum. By single-cell analysis, Pdgfrb expression was found among a subset of mesenchymal cells and universally among pericytes within the periosteum. Histologic analysis of Pdgfrβ reporter activity confirmed a combination of perivascular and non-perivascular Pdgfrβ-expressing cell types. When isolated, Pdgfrβ reporter+ skeletal periosteal cells showed enhanced colony-forming, proliferative, migratory, and osteogenic capacities. Pdgfrβ reporter+ cells were further distinguished by co-expression of the pericyte marker CD146, which yielded Pdgfrβ+CD146+ pericytes and Pdgfrβ+CD146- skeletal mesenchymal cells. Colony forming and proliferative capacity were most highly enriched among Pdgfrβ+CD146+ pericytes, while osteogenic differentiation was similarly enriched across both Pdgfrβ+ cell fractions. In summary, Pdgfrβ expression identifies multiple subsets of progenitor cells within long bone periosteum with or without perivascular distribution and with overlapping cellular features.

The potential key genes within focal adhesion that regulate mesenchymal stem cells osteogenesis or adipogenesis in microgravity related disuse osteoporosis: an integrated analysis

This study aimed to identify key genes related to focal adhesions (FA) and cells involved in osteoblast (OS) and adipocyte (AD) differentiation in osteoporosis. A mouse model of disuse osteoporosis was made by hindlimbs unloading (HLU)/Tail - suspension. Micro - CT and histological analysis were done, and differentially expressed genes (DEGs) from GSE100930 were analyzed. Soft clustering on GSE80614 OS/AD samples found FA - related candidate genes. protein-protein interaction (PPI) network and cytoHubba's Degree algorithm identified key FA - genes, validated by quantitative polymerase chain reaction (qPCR). Key OS/AD - associated cells were identified by single - cell analysis. The mouse model showed decreased bone density, microstructure damage, increased marrow adiposity, and altered gene expression. Key FA - related genes for osteogenesis (ITGB3, LAMC1, COL6A3, ITGA8, PDGFRB) and adipogenesis (ITGB3, ITGA4, LAMB1, ITGA8, LAMA4) were found and validated. Key cells (chondrocyte, adipocyte, and osteoblast progenitors) are involved in specific pathways, with osteoblast progenitors having stronger interactions. Pseudotime analysis implies differentiation from chondrocyte progenitors to adipocyte, then osteoblast progenitors. This study provides new insights for disuse osteoporosis research.

Apolipoprotein E is a marker of all chondrocytes in the growth plate resting zone

The resting zone (RZ) in mammalian growth plates is critical for maintaining and regulating chondrocyte turnover during longitudinal bone growth as a control tower and stem cell reservoir. Although recent lineage tracing studies have identified several markers for stem cells in the RZ, these markers only partially label chondrocytes in the RZ, suggesting that the resting chondrocytes (RCs) are a heterogeneous population with different types of stem cells. Since a comprehensive marker for RCs is still lacking, the RZ is generally determined based on ambiguous histological criteria, such as small and round chondrocytes without columnar formation, which may lead to inconsistencies among researchers. Therefore, in this study, we used single-cell RNA sequencing (scRNAseq) of growth plate chondrocytes followed by validation by fluorescence in situ hybridization (FISH) to precisely annotate cell clusters in scRNAseq and search for a marker of RCs. The scRNAseq analysis revealed that apolipoprotein E (Apoe) was the top-hit gene, which was ubiquitously expressed in the RC cluster. FISH confirmed that Apoe was exclusively localized to the histologically defined RZ. In newly generated ApoemCherry knock-in mice, we further confirmed that mCherry expression mirrored the distribution of Apoe-expressing chondrocytes in the RZ particularly after the formation of the secondary ossification center. These mCherry+ RCs were slow cycling in vivo and exhibited stem cell properties in vitro. Moreover, APOE was detected in human growth plate RCs. These findings suggest that apolipoprotein E is a novel pan-RC marker in both mouse and human growth plates.

Clinical challenges in bone tissue engineering - A narrative review

Bone tissue engineering (BTE) has emerged as a promising approach to address large bone defects caused by trauma, infections, congenital malformations, and tumors. This review focuses on scaffold design, cell sources, growth factors, and vascularization strategies, highlighting their roles in developing effective treatments. We explore the complexities of balancing mechanical properties, porosity, and biocompatibility in scaffold materials, alongside optimizing mesenchymal stem cell delivery methods. The critical role of growth factors in bone regeneration and the need for controlled release systems are discussed. Vascularization remains a significant hurdle, with strategies such as angiogenic factors, co-culture systems, and bioprinting under investigation. Mechanical challenges, tissue responses, and inflammation management are examined, alongside gene therapy's potential for enhancing osteogenesis and angiogenesis via both viral and non-viral delivery methods. The review emphasizes the impact of patient-specific factors on bone healing outcomes and the importance of personalized approaches. Future directions are described, emphasizing the necessity of interdisciplinary cooperation to advance the field of BTE and convert laboratory results into clinically feasible solutions.

Skeletal stem and progenitor cells in bone physiology, ageing and disease

Skeletal stem cells (SSCs) and related progenitors with osteogenic potential, collectively termed skeletal stem and/or progenitor cells (SSPCs), are crucial for providing osteoblasts for bone formation during homeostatic tissue turnover and fracture repair. Besides mediating normal bone physiology, they also have important roles in various metabolic bone diseases, including osteoporosis. SSPCs are of tremendous interest because they represent prime future targets for osteoanabolic therapies and bone regenerative medicine. Remarkable progress has been made in characterizing various SSC and SSPC populations in postnatal bone. SSPCs exist in the periosteum and within the bone marrow stroma, including subsets localizing around arteriolar and sinusoidal blood vessels; they can display osteogenic, chondrogenic, adipogenic and/or fibroblastic potential, and exert critical haematopoiesis-supportive functions. However, much remains to be clarified. By the current markers, bona fide SSCs are commonly contained within broader SSPC populations characterized by considerable heterogeneity and overlap, whose common versus specific functions in health and disease have not been fully unravelled. Here, we review the present knowledge of the identity, fates and relationships of SSPC populations in the postnatal bone environment, their contributions to bone maintenance, the changes observed upon ageing, and the effect of metabolic diseases such as osteoporosis and diabetes mellitus.

A Procedural Overview of the Involvement of Small Molecules in the Nervous System in the Regulation of Bone Healing

Clinically, a multitude of factors can contribute to the development of bone defects. In the process of bone healing, the nervous system plays a vital role in bone regeneration. Small molecules from the nervous system, such as neurotrophic factors and neuropeptides, have been found to stimulate osteoblast proliferation and differentiation by activating signaling pathways associated with bone calcification and angiogenesis. These small molecules play a crucial regulatory role at various stages of bone healing. The systematic release mechanism of small molecules within the nervous system through diverse bone tissue engineering materials holds significant clinical implications for the controlled regulation of the bone healing process. This review provides an overview of the involvement of various nervous system small molecules at different stages of bone healing and discusses their regulatory mechanisms, aiming to establish a theoretical foundation for programmed regulation in bone regeneration and design of replacement materials in bone tissue engineering.

Single-nucleus transcriptomics reveal the differentiation trajectories of periosteal skeletal/stem progenitor cells in bone regeneration

Bone regeneration is mediated by skeletal stem/progenitor cells (SSPCs) that are mainly recruited from the periosteum after bone injury. The composition of the periosteum and the steps of SSPC activation and differentiation remain poorly understood. Here, we generated a single-nucleus atlas of the periosteum at steady state and of the fracture site during the early stages of bone repair (https://fracture-repair-atlas.cells.ucsc.edu). We identified periosteal SSPCs expressing stemness markers (Pi16 and Ly6a/SCA1) and responding to fracture by adopting an injury-induced fibrogenic cell (IIFC) fate, prior to undergoing osteogenesis or chondrogenesis. We identified distinct gene cores associated with IIFCs and their engagement into osteogenesis and chondrogenesis involving Notch, Wnt, and the circadian clock signaling, respectively. Finally, we show that IIFCs are the main source of paracrine signals in the fracture environment, suggesting a crucial paracrine role of this transient IIFC population during fracture healing. Overall, our study provides a complete temporal topography of the early stages of fracture healing and the dynamic response of periosteal SSPCs to injury, redefining our knowledge of bone regeneration.

Progranulin-dependent repair function of regulatory T cells drives bone-fracture healing

Local immunoinflammatory events instruct skeletal stem cells (SSCs) to repair/regenerate bone after injury, but mechanisms are incompletely understood. We hypothesized that specialized Tregs are necessary for bone repair and interact directly with SSCs through organ-specific messages. Both in human patients with bone fracture and a mouse model of bone injury, we identified a bone injury-responding Treg subpopulation with bone-repair capacity marked by CCR8. Local production of CCL1 induced a massive migration of CCR8+ Tregs from periphery to the injury site. Depending on secretion of progranulin (PGRN), a protein encoded by the granulin (Grn) gene, CCR8+ Tregs supported the accumulation and osteogenic differentiation of SSCs and thereby bone repair. Mechanistically, we revealed that CCL1 enhanced expression levels of basic leucine zipper ATF-like transcription factor (BATF) in CCR8+ Tregs, which bound to the Grn promoter and increased Grn translational output and then PGRN secretion. Together, our work provides a new perspective in osteoimmunology and highlights possible ways of manipulating Treg signaling to enhance bone repair and regeneration.

Type-H endothelial cell protein Clec14a orchestrates osteoblast activity during trabecular bone formation and patterning

Type-H capillary endothelial cells control bone formation during embryogenesis and postnatal growth but few signalling mechanisms underpinning this influence have been characterised. Here, we identify a highly expressed type-H endothelial cell protein, Clec14a, and explore its role in coordinating osteoblast activity. Expression of Clec14a and its ligand, Mmrn2 are high in murine type-H endothelial cells but absent from osteoblasts. Clec14a-/- mice have premature condensation of the type-H vasculature and expanded distribution of osteoblasts and bone matrix, increased long-bone length and bone density indicative of accelerated skeletal development, and enhanced osteoblast maturation. Antibody-mediated blockade of the Clec14a-Mmrn2 interaction recapitulates the Clec14a-/- phenotype. Endothelial cell expression of Clec14a regulates osteoblast maturation and mineralisation activity during postnatal bone development in mice. This finding underscores the importance of type-H capillary control of osteoblast activity in bone formation and identifies a novel mechanism that mediates this vital cellular crosstalk.

Apolipoprotein E is a novel marker for chondrocytes in the growth plate resting zone

The resting zone (RZ) in mammalian growth plates is critical for maintaining and regulating chondrocyte turnover during longitudinal bone growth as a control tower and stem cell reservoir. Although recent lineage tracing studies have identified several markers for stem cells in the RZ, these markers only partially label chondrocytes in the RZ, suggesting that the resting chondrocytes (RCs) are a heterogeneous population with different types of stem cells. Since a comprehensive marker for RCs is still lacking, the RZ is generally determined based on ambiguous histological criteria, such as small and round chondrocytes without columnar formation, which may lead to inconsistencies among researchers. Therefore, in this study, we used single-cell RNA sequencing (scRNAseq) of growth plate chondrocytes followed by validation by fluorescence in situ hybridization (FISH) to precisely annotate cell clusters in scRNAseq and search for a marker of RCs. The scRNAseq analysis revealed that apolipoprotein E (Apoe) was the top-hit gene, which was ubiquitously expressed in the RC cluster. FISH confirmed that Apoe was exclusively localized to the histologically defined RZ. In newly generated Apoe-mCherry knock-in mice, we further confirmed that mCherry expression mirrored the distribution of Apoe-expressing chondrocytes in the RZ particularly after the formation of the secondary ossification center. These mCherry+ RCs were slow cycling in vivo and exhibited stem cell properties both in vitro and in vivo. Moreover, APOE was detected in human growth plate RCs. These findings suggest that Apoe is a novel pan-RC marker in both mouse and human growth plates.

Targeting endothelial PDGFR-β facilitates angiogenesis-associated bone formation through the PAK1/NICD axis

The intricate orchestration of osteoporosis (OP) pathogenesis remains elusive. Mounting evidence suggests that angiogenesis-driven osteogenesis serves as a crucial foundation for maintaining bone homeostasis. This study aimed to explore the potential of the endothelial platelet-derived growth factor receptor-β (PDGFR-β) in mitigating bone loss through its facilitation of H-type vessel formation. Our findings demonstrate that the expression level of endothelial PDGFR-β is reduced in samples obtained from individuals suffering from OP, as well as in ovariectomy mice. Depletion of PDGFR-β in endothelial cells ameliorates angiogenesis-mediated bone formation in mice. The regulatory influence of endothelial PDGFR-β on H-type vessels is mediated through the PDGFRβ-P21-activated kinase 1-Notch1 intracellular domain signaling cascade. In particular, the endothelium-specific enhancement of PDGFR-β facilitates H-type vessels and their associated bone formation in OP. Hence, the strategic targeting of endothelial PDGFR-β emerges as a promising therapeutic approach for the management of OP in the near future.

Single nuclei transcriptomics reveal the differentiation trajectories of periosteal skeletal/stem progenitor cells in bone regeneration

Bone regeneration is mediated by skeletal stem/progenitor cells (SSPCs) that are mainly recruited from the periosteum after bone injury. The composition of the periosteum and the steps of SSPC activation and differentiation remain poorly understood. Here, we generated a single-nuclei atlas of the periosteum at steady-state and of the fracture site during early stages of bone repair ( https://fracture-repair-atlas.cells.ucsc.edu ). We identified periosteal SSPCs expressing stemness markers ( Pi16 and Ly6a /SCA1) and responding to fracture by adopting an injury-induced fibrogenic cell (IIFC) fate, prior to undergoing osteogenesis or chondrogenesis. We identified distinct gene cores associated with IIFCs and their engagement into osteogenesis and chondrogenesis involving Notch, Wnt and the circadian clock signaling respectively. Finally, we show that IIFCs are the main source of paracrine signals in the fracture environment, suggesting a crucial paracrine role of this transient IIFC population during fracture healing. Overall, our study provides a complete temporal topography of the early stages of fracture healing and the dynamic response of periosteal SSPCs to injury, redefining our knowledge of bone regeneration.

Skeletal stem and progenitor cells in bone development and repair

Bone development, growth, and repair are complex processes involving various cell types and interactions, with central roles played by skeletal stem and progenitor cells. Recent research brought new insights into the skeletal precursor populations that mediate intramembranous and endochondral bone development. Later in life, many of the cellular and molecular mechanisms determining development are reactivated upon fracture, with powerful trauma-induced signaling cues triggering a variety of postnatal skeletal stem/progenitor cells (SSPCs) residing near the bone defect. Interestingly, in this injury context, the current evidence suggests that the fates of both SSPCs and differentiated skeletal cells can be considerably flexible and dynamic, and that multiple cell sources can be activated to operate as functional progenitors generating chondrocytes and/or osteoblasts. The combined implementation of in vivo lineage tracing, cell surface marker-based cell selection, single-cell molecular analyses, and high-resolution in situ imaging has strongly improved our insights into the diversity and roles of developmental and reparative stem/progenitor subsets, while also unveiling the complexity of their dynamics, hierarchies, and relationships. Albeit incompletely understood at present, findings supporting lineage flexibility and possibly plasticity among sources of osteogenic cells challenge the classical dogma of a single primitive, self-renewing, multipotent stem cell driving bone tissue formation and regeneration from the apex of a hierarchical and strictly unidirectional differentiation tree. We here review the state of the field and the newest discoveries in the origin, identity, and fates of skeletal progenitor cells during bone development and growth, discuss the contributions of adult SSPC populations to fracture repair, and reflect on the dynamism and relationships among skeletal precursors and differentiated cell lineages. Further research directed at unraveling the heterogeneity and capacities of SSPCs, as well as the regulatory cues determining their fate and functioning, will offer vital new options for clinical translation toward compromised fracture healing and bone regenerative medicine.

Angiogenesis is uncoupled from osteogenesis during calvarial bone regeneration

Bone regeneration requires a well-orchestrated cellular and molecular response including robust vascularization and recruitment of mesenchymal and osteogenic cells. In femoral fractures, angiogenesis and osteogenesis are closely coupled during the complex healing process. Here, we show with advanced longitudinal intravital multiphoton microscopy that early vascular sprouting is not directly coupled to osteoprogenitor invasion during calvarial bone regeneration. Early osteoprogenitors emerging from the periosteum give rise to bone-forming osteoblasts at the injured calvarial bone edge. Microvessels growing inside the lesions are not associated with osteoprogenitors. Subsequently, osteogenic cells collectively invade the vascularized and perfused lesion as a multicellular layer, thereby advancing regenerative ossification. Vascular sprouting and remodeling result in dynamic blood flow alterations to accommodate the growing bone. Single cell profiling of injured calvarial bones demonstrates mesenchymal stromal cell heterogeneity comparable to femoral fractures with increase in cell types promoting bone regeneration. Expression of angiogenesis and hypoxia-related genes are slightly elevated reflecting ossification of a vascularized lesion site. Endothelial Notch and VEGF signaling alter vascular growth in calvarial bone repair without affecting the ossification progress. Our findings may have clinical implications for bone regeneration and bioengineering approaches.

2-N, 6-O sulfated chitosan evokes periosteal stem cells for bone regeneration

Musculoskeletal injuries and bone defects represent a significant clinical challenge, necessitating innovative approaches for effective bone tissue regeneration. In this study, we investigated the potential of harnessing periosteal stem cells (PSCs) and glycosaminoglycan (GAG)-mimicking materials for in situ bone regeneration. Our findings demonstrated that the introduction of 2-N, 6-O sulfated chitosan (26SCS), a GAG-like polysaccharide, enriched PSCs and promoted robust osteogenesis at the defect area. Mechanistically, 26SCS amplifies the biological effect of endogenous platelet-derived growth factor-BB (PDGF-BB) through enhancing the interaction between PDGF-BB and its receptor PDGFRβ abundantly expressed on PSCs, resulting in strengthened PSC proliferation and osteogenic differentiation. As a result, 26SCS effectively improved bone defect repair, even in an osteoporotic mouse model with lowered PDGF-BB level and diminished regenerative potential. Our findings suggested the significant potential of GAG-like biomaterials in regulating PSC behavior, which holds great promise for addressing osteoporotic bone defect repair in future applications.

Mechanistic insights into the spontaneous induction of bone formation

The grand discovery of morphogens, or "form-generating substances", revealed that tissue morphogenesis is initiated by soluble molecular signals or morphogens primarily belonging to the transforming growth factor-β (TGF-β) supergene family. The regenerative potential of bone rests on its extracellular matrix, which is the repository of several morphogens that tightly control cellular differentiating pathways, cellular matrix deposition and remodeling. Alluringly, the matrix also contains specific factors transferred from the heterotopic implanted bone matrices initiating "Tissue Induction", as provocatively described in Nature in 1945. Later, it was found that selected genes and gene products of the TGF-β supergene family singly, synchronously, and synergistically mastermind the induction of bone formation. This review describes the phenomenon of the spontaneous and/or intrinsic osteoinductivity of calcium phosphate-based biomaterials and titanium' constructs without the applications of soluble osteogenetic molecular signals. The review shows the spontaneous induction of bone formation initiated by Ca++ activating stem cell differentiation and up-regulation of bone morphogenetic proteins genes. Expressed gene products are embedded into the concavities of the calcium phosphate-based substrata, initiating bone formation as a secondary response. Pure titanium's substrata do not initiate the spontaneous induction of bone formation. The induction of bone is solely dependent on acid, alkali and heat treatments to form apatite layers on the treated titanium surfaces. The induction of bone formation is achieved exclusively by apatite-based biomaterial surfaces. The hydroxyapatite, in its various forms and geometric configurations, finely tunes the induction of bone formation in heterotopic sites. Cellular differentiation by fine-tuning of the cellular molecular machinery is initiated by specific geometric modularity of the hydroxyapatite substrata that push cellular buttons that start the ripple-like cascade of "Tissue Induction", generating newly formed ossicles with bone marrow in heterotopic extraskeletal sites. The highlighted mechanistic insights into the spontaneous induction of bone formation are a research platform invocating selected molecular elements to construct the induction of bone formation.

Periosteum Containing Implicit Stem Cells: A Progressive Source of Inspiration for Bone Tissue Regeneration

The periosteum is known as the thin connective tissue covering most bone surfaces. Its extrusive bone regeneration capacity was confirmed from the very first century-old studies. Recently, pluripotent stem cells in the periosteum with unique physiological properties were unveiled. Existing in dynamic contexts and regulated by complex molecular networks, periosteal stem cells emerge as having strong capabilities of proliferation and multipotential differentiation. Through continuous exploration of studies, we are now starting to acquire more insight into the great potential of the periosteum in bone formation and repair in situ or ectopically. It is undeniable that the periosteum is developing further into a more promising strategy to be harnessed in bone tissue regeneration. Here, we summarized the development and structure of the periosteum, cell markers, and the biological features of periosteal stem cells. Then, we reviewed their pivotal role in bone repair and the underlying molecular regulation. The understanding of periosteum-related cellular and molecular content will help enhance future research efforts and application transformation of the periosteum.

Blood and bones: Mechanical cues and Hippo signaling drive vascular invasion during limb formation

Although mechanical cues are known to influence the postnatal skeleton, the impact of bone cell mechano-transduction on early skeletal development remains less clear. In this issue of Developmental Cell, Collins et al. (2023) report that YAP/TAZ deletion in osteoblast precursors reduces Cxcl12 expression, leading to defects in bone vascularization.

YAP and TAZ couple osteoblast precursor mobilization to angiogenesis and mechanoregulation in murine bone development

Fetal bone development occurs through the conversion of avascular cartilage to vascularized bone at the growth plate. This requires coordinated mobilization of osteoblast precursors with blood vessels. In adult bone, vessel-adjacent osteoblast precursors are maintained by mechanical stimuli; however, the mechanisms by which these cells mobilize and respond to mechanical cues during embryonic development are unknown. Here, we show that the mechanoresponsive transcriptional regulators Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) spatially couple osteoblast precursor mobilization to angiogenesis, regulate vascular morphogenesis to control cartilage remodeling, and mediate mechanoregulation of embryonic murine osteogenesis. Mechanistically, YAP and TAZ regulate a subset of osteoblast-lineage cells, identified by single-cell RNA sequencing as vessel-associated osteoblast precursors, which regulate transcriptional programs that direct blood vessel invasion through collagen-integrin interactions and Cxcl12. Functionally, in 3D human cell co-culture, CXCL12 treatment rescues angiogenesis impaired by stromal cell YAP/TAZ depletion. Together, these data establish functions of the vessel-associated osteoblast precursors in bone development.

Pericytes as the Orchestrators of Vasculature and Adipogenesis

Pericytes (PCs) are located surrounding the walls of small blood vessels, particularly capillaries and microvessels. In addition to their functions in maintaining vascular integrity, participating in angiogenesis, and regulating blood flow, PCs also serve as a reservoir for multi-potent stem/progenitor cells in white, brown, beige, and bone marrow adipose tissues. Due to the complex nature of this cell population, the identification and characterization of PCs has been challenging. A comprehensive understanding of the heterogeneity of PCs may enhance their potential as therapeutic targets for metabolic syndromes or bone-related diseases. This mini-review summarizes multiple PC markers commonly employed in lineage-tracing studies, with an emphasis on their contribution to adipogenesis and functions in different adipose depots under diverse metabolic conditions.

Activin A marks a novel progenitor cell population during fracture healing and reveals a therapeutic strategy

Insufficient bone fracture repair represents a major clinical and societal burden and novel strategies are needed to address it. Our data reveal that the transforming growth factor-β superfamily member Activin A became very abundant during mouse and human bone fracture healing but was minimally detectable in intact bones. Single-cell RNA-sequencing revealed that the Activin A-encoding gene Inhba was highly expressed in a unique, highly proliferative progenitor cell (PPC) population with a myofibroblast character that quickly emerged after fracture and represented the center of a developmental trajectory bifurcation producing cartilage and bone cells within callus. Systemic administration of neutralizing Activin A antibody inhibited bone healing. In contrast, a single recombinant Activin A implantation at fracture site in young and aged mice boosted: PPC numbers; phosphorylated SMAD2 signaling levels; and bone repair and mechanical properties in endochondral and intramembranous healing models. Activin A directly stimulated myofibroblastic differentiation, chondrogenesis and osteogenesis in periosteal mesenchymal progenitor culture. Our data identify a distinct population of Activin A-expressing PPCs central to fracture healing and establish Activin A as a potential new therapeutic tool.

Fracture healing research: Recent insights

Bone has the rare capability of scarless regeneration that enables the complete restoration of the injured bone area. In recent decades, promising new technologies have emerged from basic, translational and clinical research for fracture treatment; however, 5-10 % of all bone fractures still fail to heal successfully or heal in a delayed manner. Several comorbidities and risk factors have been identified which impair bone healing and might lead to delayed bone union or non-union. Therefore, a considerable amount of research has been conducted to elucidate molecular mechanisms of successful and delayed fracture healing to gain further insights into this complex process. One focus of recent research is to investigate the complex interactions of different cell types and the action of progenitor cells during the healing process. Of particular interest is also the identification of patient-specific comorbidities and how these affect fracture healing. In this review, we discuss the recent knowledge about progenitor cells for long bone repair and the influence of comorbidities such as diabetes, postmenopausal osteoporosis, and chronic stress on the healing process. The topic selection for this review was made based on the presented studies at the 2022 annual meeting of the European Calcified Tissue Society (ECTS) in Helsinki.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Spatial transcriptomic interrogation of the murine bone marrow signaling landscape

Self-renewal and differentiation of skeletal stem and progenitor cells (SSPCs) are tightly regulated processes, with SSPC dysregulation leading to progressive bone disease. While the application of single-cell RNA sequencing (scRNAseq) to the bone field has led to major advancements in our understanding of SSPC heterogeneity, stem cells are tightly regulated by their neighboring cells which comprise the bone marrow niche. However, unbiased interrogation of these cells at the transcriptional level within their native niche environment has been challenging. Here, we combined spatial transcriptomics and scRNAseq using a predictive modeling pipeline derived from multiple deconvolution packages in adult mouse femurs to provide an endogenous, in vivo context of SSPCs within the niche. This combined approach localized SSPC subtypes to specific regions of the bone and identified cellular components and signaling networks utilized within the niche. Furthermore, the use of spatial transcriptomics allowed us to identify spatially restricted activation of metabolic and major morphogenetic signaling gradients derived from the vasculature and bone surfaces that establish microdomains within the marrow cavity. Overall, we demonstrate, for the first time, the feasibility of applying spatial transcriptomics to fully mineralized tissue and present a combined spatial and single-cell transcriptomic approach to define the cellular components of the stem cell niche, identify cell‒cell communication, and ultimately gain a comprehensive understanding of local and global SSPC regulatory networks within calcified tissue.

Testosterone Restores Body Composition, Bone Mass, and Bone Strength Following Early Puberty Suppression in a Mouse Model Mimicking the Clinical Strategy in Trans Boys

Transgender youth increasingly present at pediatric gender services. Some of them receive long-term puberty suppression with gonadotropin-releasing hormone analogues (GnRHa) before starting gender-affirming hormones (GAH). The impact of GnRHa use started in early puberty on bone composition and bone mass accrual is unexplored. It is furthermore unclear whether subsequent GAH fully restore GnRHa effects and whether the timing of GAH introduction matters. To answer these questions, we developed a mouse model mimicking the clinical strategy applied in trans boys. Prepubertal 4-week-old female mice were treated with GnRHa alone or with GnRHa supplemented with testosterone (T) from 6 weeks (early puberty) or 8 weeks (late puberty) onward. Outcomes were analyzed at 16 weeks and compared with untreated mice of both sexes. GnRHa markedly increased total body fat mass, decreased lean body mass, and had a modest negative impact on grip strength. Both early and late T administration shaped body composition to adult male levels, whereas grip strength was restored to female values. GnRHa-treated animals showed lower trabecular bone volume and reduced cortical bone mass and strength. These changes were reversed by T to female levels (cortical bone mass and strength) irrespective of the time of administration or even fully up to adult male control values (trabecular parameters) in case of earlier T start. The lower bone mass in GnRHa-treated mice was associated with increased bone marrow adiposity, also reversed by T. In conclusion, prolonged GnRHa use started in prepubertal female mice modifies body composition toward more fat and less lean mass and impairs bone mass acquisition and strength. Subsequent T administration counteracts GnRHa impact on these parameters, shaping body composition and trabecular parameters to male values while restoring cortical bone architecture and strength up to female but not male control levels. These findings could help guide clinical strategies in transgender care. © 2023 American Society for Bone and Mineral Research (ASBMR).

Cellular dynamics of distinct skeletal cells and the development of osteosarcoma

Bone contributes to the maintenance of vital biological activities. At the cellular level, multiple types of skeletal cells, including skeletal stem and progenitor cells (SSPCs), osteoblasts, chondrocytes, marrow stromal cells, and adipocytes, orchestrate skeletal events such as development, aging, regeneration, and tumorigenesis. Osteosarcoma (OS) is a primary malignant tumor and the main form of bone cancer. Although it has been proposed that the cellular origins of OS are in osteogenesis-related skeletal lineage cells with cancer suppressor gene mutations, its origins have not yet been fully elucidated because of a poor understanding of whole skeletal cell diversity and dynamics. Over the past decade, the advent and development of single-cell RNA sequencing analyses and mouse lineage-tracing approaches have revealed the diversity of skeletal stem and its lineage cells. Skeletal stem cells (SSCs) in the bone marrow endoskeletal region have now been found to efficiently generate OS and to be robust cells of origin under p53 deletion conditions. The identification of SSCs may lead to a more limited redefinition of bone marrow mesenchymal stem/stromal cells (BM-MSCs), and this population has been thought to contain cells from which OS originates. In this mini-review, we discuss the cellular diversity and dynamics of multiple skeletal cell types and the origin of OS in the native in vivo environment in mice. We also discuss future challenges in the study of skeletal cells and OS.

PDGFR in PDGF-BB/PDGFR Signaling Pathway Does Orchestrates Osteogenesis in a Temporal Manner

Platelet-derived growth factor-BB (PDGF-BB)/platelet-derived growth factor receptor-β (PDGFR-β) pathway is conventionally considered as an important pathway to promote osteogenesis; however, recent study suggested its role during osteogenesis to be controversial. Regarding the differential functions of this pathway during 3 stages of bone healing, we hypothesized that temporal inhibition of PDGF-BB/PDGFR-β pathway could shift the proliferation/differentiation balance of skeletal stem and progenitor cells, toward osteogenic lineage, which leads to improved bone regeneration. We first validated that inhibition of PDGFR-β at late stage of osteogenic induction effectively enhanced differentiation toward osteoblasts. This effect was also replicated in vivo by showing accelerated bone formation when block PDGFR-β pathway at late stage of critical bone defect healing mediated using biomaterials. Further, we found that such PDGFR-β inhibitor-initiated bone healing was also effective in the absence of scaffold implantation when administrated intraperitoneally. Mechanistically, timely inhibition of PDGFR-β blocked extracellular regulated protein kinase 1/2 pathway, which shift proliferation/differentiation balance of skeletal stem and progenitor cell to osteogenic lineage by upregulating osteogenesis-related products of Smad to induce osteogenesis. This study offered updated understanding of the use of PDGFR-β pathway and provides new insight routes of action and novel therapeutic methods in the field of bone repair.

Type H vessel/platelet-derived growth factor receptor β+ perivascular cell disintegration is involved in vascular injury and bone loss in radiation-induced bone damage

Collapse of the microvascular system is a prerequisite for radiation-induced bone loss. Since type H vessels, a specific bone vessel subtype surrounded by platelet-derived growth factor receptor β⁺ (PDGFRβ⁺ ) perivascular cells (PVCs), has been recently identified to couple angiogenesis and osteogenesis, we hypothesize that type H vessel injury initiates PDGFRβ⁺ PVC dysfunction, which contributes to the abnormal angiogenesis and osteogenesis after irradiation. In this study, we found that radiation led to the decrease of both type H endothelial cell (EC) and PDGFRβ⁺ PVC numbers. Remarkably, results from lineage tracing showed that PDGFRβ⁺ PVCs detached from microvessels and converted the lineage commitment from osteoblasts to adipocytes, leading to vascular injury and bone loss after irradiation. These phenotype transitions above were further verified to be associated with the decrease in hypoxia-inducible factor-1α (HIF-1α)/PDGF-BB/PDGFRβ signalling between type H ECs and PDGFRβ⁺ PVCs. Pharmacological blockade of HIF-1α/PDGF-BB/PDGFRβ signalling induced a phenotype similar to radiation-induced bone damage, while the rescue of this signalling significantly alleviated radiation-induced bone injury. Our findings show that the decrease in HIF-1α/PDGF-BB/PDGFRβ signalling between type H ECs and PDGFRβ⁺ PVCs after irradiation affects the homeostasis of EC-PVC coupling and plays a part in vascular damage and bone loss, which has broad implications for effective translational therapies.

YAP and TAZ couple osteoblast precursor mobilization to angiogenesis and mechanoregulated bone development

Endochondral ossification requires coordinated mobilization of osteoblast precursors with blood vessels. During adult bone homeostasis, vessel adjacent osteoblast precursors respond to and are maintained by mechanical stimuli; however, the mechanisms by which these cells mobilize and respond to mechanical cues during embryonic development are unknown. Previously, we found that deletion of the mechanoresponsive transcriptional regulators, YAP and TAZ, from Osterix-expressing osteoblast precursors and their progeny caused perinatal lethality. Here, we show that embryonic YAP/TAZ signaling couples vessel-associated osteoblast precursor mobilization to angiogenesis in developing long bones. Osterix-conditional YAP/TAZ deletion impaired endochondral ossification in the primary ossification center but not intramembranous osteogenesis in the bone collar. Single-cell RNA sequencing revealed YAP/TAZ regulation of the angiogenic chemokine, Cxcl12, which was expressed uniquely in vessel-associated osteoblast precursors. YAP/TAZ signaling spatially coupled osteoblast precursors to blood vessels and regulated vascular morphogenesis and vessel barrier function. Further, YAP/TAZ signaling regulated vascular loop morphogenesis at the chondro-osseous junction to control hypertrophic growth plate remodeling. In human cells, mesenchymal stromal cell co-culture promoted 3D vascular network formation, which was impaired by stromal cell YAP/TAZ depletion, but rescued by recombinant CXCL12 treatment. Lastly, YAP and TAZ mediated mechanotransduction for load-induced osteogenesis in embryonic bone.

PDGF inhibits BMP2-induced bone healing

Bone regeneration depends on a pool of bone/cartilage stem/progenitor cells and signaling mechanisms regulating their differentiation. Using in vitro approach, we have shown that PDGF signaling through PDGFRβ inhibits BMP2-induced osteogenesis, and significantly attenuates expression of BMP2 target genes. We evaluated outcomes of treatment with two anabolic agents, PDGF and BMP2 using different bone healing models. Targeted deletion of PDGFRβ in αSMA osteoprogenitors, led to increased callus bone mass, resulting in improved biomechanical properties of fractures. In critical size bone defects BMP2 treatment increased proportion of osteoprogenitors, while the combined treatment of PDGF BB with BMP2 decreased progenitor number at the injury site. BMP2 treatment induced significant bone formation and increased number of osteoblasts, while in contrast combined treatment with PDGF BB decreased osteoblast numbers. This is in vivo study showing that PDGF inhibits BMP2-induced osteogenesis, but inhibiting PDGF signaling early in healing process does not improve BMP2-induced bone healing.

Conditional Loss of Nmp4 in Mesenchymal Stem Progenitor Cells Enhances PTH-Induced Bone Formation

Activation of bone anabolic pathways is a fruitful approach for treating severe osteoporosis, yet FDA-approved osteoanabolics, eg, parathyroid hormone (PTH), have limited efficacy. Improving their potency is a promising strategy for maximizing bone anabolic output. Nmp4 (Nuclear Matrix Protein 4) global knockout mice exhibit enhanced PTH-induced increases in trabecular bone but display no overt baseline skeletal phenotype. Nmp4 is expressed in all tissues; therefore, to determine which cell type is responsible for driving the beneficial effects of Nmp4 inhibition, we conditionally removed this gene from cells at distinct stages of osteogenic differentiation. Nmp4-floxed (Nmp4fl/fl ) mice were crossed with mice bearing one of three Cre drivers including (i) Prx1Cre+  to remove Nmp4 from mesenchymal stem/progenitor cells (MSPCs) in long bones; (ii) BglapCre+  targeting mature osteoblasts, and (iii) Dmp1Cre+  to disable Nmp4 in osteocytes. Virgin female Cre+  and Cre- mice (10 weeks of age) were sorted into cohorts by weight and genotype. Mice were administered daily injections of either human PTH 1-34 at 30 μg/kg or vehicle for 4 weeks or 7 weeks. Skeletal response was assessed using dual-energy X-ray absorptiometry, micro-computed tomography, bone histomorphometry, and serum analysis for remodeling markers. Nmp4fl/fl ;Prx1Cre+  mice virtually phenocopied the global Nmp4-/- skeleton in the femur, ie, a mild baseline phenotype but significantly enhanced PTH-induced increase in femur trabecular bone volume/total volume (BV/TV) compared with their Nmp4fl/fl ;Prx1Cre- controls. This was not observed in the spine, where Prrx1 is not expressed. Heightened response to PTH was coincident with enhanced bone formation. Conditional loss of Nmp4 from the mature osteoblasts (Nmp4fl/fl ;BglapCre+ ) failed to increase BV/TV or enhance PTH response. However, conditional disabling of Nmp4 in osteocytes (Nmp4fl/fl ;Dmp1Cre+ ) increased BV/TV without boosting response to hormone under our experimental regimen. We conclude that Nmp4-/- Prx1-expressing MSPCs drive the improved response to PTH therapy and that this gene has stage-specific effects on osteoanabolism. © 2022 American Society for Bone and Mineral Research (ASBMR).

Polymorphic Variants of the PDGFRB Gene Influence Efficacy of PRP Therapy in Treating Tennis Elbow: A Prospective Cohort Study

BACKGROUND

Differences in response to PRP (platelet-rich plasma) therapy may be linked to the variability of growth factors and their receptor's genes. Considering that, we checked whether the platelet-derived growth factor receptor beta gene (PDGFRB) single nucleotide polymorphisms (SNPs) affect the effectiveness of PRP therapy in treating tennis elbow patients.

METHODS

The treatment efficacy was analyzed over time (2, 4, 8, 12, 24, 52, and 104 weeks after PRP injection) on 107 patients (132 elbows) using PROMs (patient-reported outcome measures), namely VAS (Visual Analog Scale), QDASH (quick version of Disabilities of the Arm, Shoulder, and Hand) and PRTEE (Patient-Rated Tennis Elbow Evaluation). Five polymorphisms of the PDGFRB gene (rs4324662, rs758588, rs3828610, rs3756311, and rs3756312) were genotyped.

RESULTS

The CC (rs3828610) and GG (rs3756311 and rs3756312) genotypes had a particularly strong impact on the effectiveness of the therapy, as measured by the values of PROMs, both in additive as well as dominant/recessive models. These homozygotes were also characterized by significantly higher values of MPV (mean platelet volume).

CONCLUSIONS

The PDGFRB gene SNPs affect the effectiveness of PRP therapy in treating tennis elbow patients and it may result from the differentiated metabolic activity of platelets in particular genotype variants.

Periosteal Skeletal Stem and Progenitor Cells in Bone Regeneration

PURPOSE OF REVIEW

The periosteum, the outer layer of bone, is a major source of skeletal stem/progenitor cells (SSPCs) for bone repair. Here, we discuss recent findings on the characterization, role, and regulation of periosteal SSPCs (pSSPCs) during bone regeneration.

RECENT FINDINGS

Several markers have been described for pSSPCs but lack tissue specificity. In vivo lineage tracing and transcriptomic analyses have improved our understanding of pSSPC functions during bone regeneration. Bone injury activates pSSPCs that migrate, proliferate, and have the unique potential to form both bone and cartilage. The injury response of pSSPCs is controlled by many signaling pathways including BMP, FGF, Notch, and Wnt, their metabolic state, and their interactions with the blood clot, nerve fibers, blood vessels, and macrophages in the fracture environment. Periosteal SSPCs are essential for bone regeneration. Despite recent advances, further studies are required to elucidate pSSPC heterogeneity and plasticity that make them a central component of the fracture healing process and a prime target for clinical applications.

VEGFA from osteoblasts is not required for lamellar bone formation following tibial loading

The relationship between osteogenesis and angiogenesis is complex. Normal bone development requires angiogenesis, mediated by vascular endothelial growth factor A (VEGFA). Studies have demonstrated through systemic inhibition or genetic modification that VEGFA is indispensable for several types of bone repair, presumably via its role in supporting angiogenesis. But a direct role for VEGFA within osteoblasts, in the absence of angiogenesis, has also been suggested. To address the question of whether VEGFA from osteoblasts supports bone formation directly, we applied anabolic loading to induce lamellar bone formation in mice, a process shown to be independent of angiogenesis. We hypothesized that VEGFA from osteoblasts is required for lamellar bone formation. To test this hypothesis, we applied axial tibial compression to inducible Cre/LoxP mice from three lines. Vegfafl/fl mice were crossed with Ubiquitin C (UBC), Osterix (Osx) and Dentin-Matrix Protein 1 (DMP1) Cre-ERT2 mice to target all cells, (pre)osteoblast-lineage cells, and mature osteoblasts and osteocytes, respectively. Genotype effects were determined by comparing control (Vegfafl/fl) and Cre+ (VegfaΔ) mice for each line. At 5 months of age tamoxifen was injected for 5 days followed by a 3-week clearance prior to loading. Female and male mice (N = 100) were loaded for 5 days to peak forces to engender -3100 με peak compressive strain and processed for dynamic histomorphometry (day 12). Percent MS/BS increased 20-70 % as a result of loading, with no effect of genotype in Osx or Dmp1 lines. In contrast, the UBC groups had a significant decrease in relative periosteal BFR/BS in VegfaΔ vs. Vegfafl/fl mice. The UBC line did not have any cortical bone phenotype in non-loaded femurs. In summary, dynamic histomorphometry data confirmed that tibial loading induces lamellar bone formation. Contrary to our hypothesis, there was no decrease in loading-induced bone formation in the Osx or Dmp1 lines in the absence of VEGFA. There was a decrease in bone formation in the UBC line where all cells were targeted. This result indicates that VEGFA from a non-osteoblast cell source supports loading-induced lamellar bone formation, although osteoblast/osteocyte VEGFA is dispensable. These findings support a paracrine model whereby non-osteoblast VEGFA supports lamellar bone formation, independent of angiogenesis.

Skeletal Stem/Progenitor Cells in Periosteum and Skeletal Muscle Share a Common Molecular Response to Bone Injury

Bone regeneration involves skeletal stem/progenitor cells (SSPCs) recruited from bone marrow, periosteum, and adjacent skeletal muscle. To achieve bone reconstitution after injury, a coordinated cellular and molecular response is required from these cell populations. Here, we show that SSPCs from periosteum and skeletal muscle are enriched in osteochondral progenitors, and more efficiently contribute to endochondral ossification during fracture repair as compared to bone-marrow stromal cells. Single-cell RNA sequencing (RNAseq) analyses of periosteal cells reveal the cellular heterogeneity of periosteum at steady state and in response to bone fracture. Upon fracture, both periosteal and skeletal muscle SSPCs transition from a stem/progenitor to a fibrogenic state prior to chondrogenesis. This common activation pattern in periosteum and skeletal muscle SSPCs is mediated by bone morphogenetic protein (BMP) signaling. Functionally, Bmpr1a gene inactivation in platelet-derived growth factor receptor alpha (Pdgfra)-derived SSPCs impairs bone healing and decreases SSPC proliferation, migration, and osteochondral differentiation. These results uncover a coordinated molecular program driving SSPC activation in periosteum and skeletal muscle toward endochondral ossification during bone regeneration. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Therapeutic Efficacy and Safety of Osteoinductive Factors and Cellular Therapies for Long Bone Fractures and Non-Unions: A Meta-Analysis and Systematic Review

BACKGROUND

Long bone fractures display significant non-union rates, but the exact biological mechanisms implicated in this devastating complication remain unclear. The combination of osteogenetic and angiogenetic factors at the fracture site is an essential prerequisite for successful bone regeneration. The aim of this study is to investigate the results of the clinical implantation of growth factors for intraoperative enhancement of osteogenesis for the treatment of long bone fractures and non-unions.

METHODS

A systematic literature review search was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines in the PubMed and Web of Science databases from the date of inception of each database through to 10 January 2022. Specific inclusion and exclusion criteria were applied in order to identify relevant studies reporting on the treatment of upper and lower limb long bone non-unions treated with osteoinductive or cellular factors.

RESULTS

Overall, 18 studies met the inclusion criteria and examined the effectiveness of the application of Bone Morphogenetic Proteins-2 and -7 (BMPs), platelet rich plasma (PRP) and mesenchymal stem cells (MSCs). Despite the existence of limitations in the studies analysed (containing mixed groups of open and close fractures, different types of fractures, variability of treatment protocols, different selection criteria and follow-up periods amongst others), their overall effectiveness was found significantly increased in patients who received them compared with the controls (I2 = 60%, 95% CI = 1.59 [0.99-2.54], Z =1.93, p = 0.05).

CONCLUSION

Administration of BMP-2 and -7, PRP and MSCs were considered effective and safe methods in fracture treatment, increasing bone consolidation, reducing time to repair and being linked to satisfactory postoperative functional scores.

Penttinen syndrome-associated PDGFRB Val665Ala variant causes aberrant constitutive STAT1 signalling

Penttinen syndrome is a rare progeroid disorder caused by mutations in platelet‐derived growth factor (PDGF) receptor beta (encoded by the PDGFRB proto‐oncogene) and characterized by a prematurely aged appearance with lipoatrophy, skin lesions, thin hair and acro‐osteolysis. Activating mutations in PDGFRB have been associated with other human diseases, including Kosaki overgrowth syndrome, infantile myofibromatosis, fusiform aneurysms, acute lymphoblastic leukaemia and myeloproliferative neoplasms associated with eosinophilia. The goal of the present study was to characterize the PDGFRB p.Val665Ala variant associated with Penttinen syndrome at the molecular level. This substitution is located in a conserved loop of the receptor tyrosine kinase domain. We observed that the mutant receptor was expressed at a lower level but showed constitutive activity. In the absence of ligand, the mutant activated STAT1 and elicited an interferon‐like transcriptional response. Phosphorylation of STAT3, STAT5, AKT and phospholipase Cγ was weak or undetectable. It was devoid of oncogenic activity in two cell proliferation assays, contrasting with classical PDGF receptor oncogenic mutants. STAT1 activation was not sensitive to ruxolitinib and did not rely on interferon‐JAK2 signalling. Another tyrosine kinase inhibitor, imatinib, blocked signalling by the p.Val665Ala variant at a higher concentration compared with the wild‐type receptor. Importantly, this concentration remained in the therapeutic range. Dasatinib, nilotinib and ponatinib also inhibited the mutant receptor. In conclusion, the p.Val665Ala variant confers unique features to PDGF receptor β compared with other characterized gain‐of‐function mutants, which may in part explain the particular set of symptoms associated with Penttinen syndrome.

Insights Into the Role of Platelet-Derived Growth Factors: Implications for Parkinson’s Disease Pathogenesis and Treatment

Parkinson’s disease (PD), the second most common neurodegenerative disease after Alzheimer’s disease, commonly occurs in the elderly population, causing a significant medical and economic burden to the aging society worldwide. At present, there are few effective methods that achieve satisfactory clinical results in the treatment of PD. Platelet-derived growth factors (PDGFs) and platelet-derived growth factor receptors (PDGFRs) are important neurotrophic factors that are expressed in various cell types. Their unique structures allow for specific binding that can effectively regulate vital functions in the nervous system. In this review, we summarized the possible mechanisms by which PDGFs/PDGFRs regulate the occurrence and development of PD by affecting oxidative stress, mitochondrial function, protein folding and aggregation, Ca2+ homeostasis, and cell neuroinflammation. These modes of action mainly depend on the type and distribution of PDGFs in different nerve cells. We also summarized the possible clinical applications and prospects for PDGF in the treatment of PD, especially in genetic treatment. Recent advances have shown that PDGFs have contradictory roles within the central nervous system (CNS). Although they exert neuroprotective effects through multiple pathways, they are also associated with the disruption of the blood–brain barrier (BBB). Our recommendations based on our findings include further investigation of the contradictory neurotrophic and neurotoxic effects of the PDGFs acting on the CNS.

SM22α-lineage niche cells regulate intramembranous bone regeneration via PDGFRβ-triggered hydrogen sulfide production

Bone stromal cells are critical for bone homeostasis and regeneration. Growing evidence suggests that non-stem bone niche cells support bone homeostasis and regeneration via paracrine mechanisms, which remain to be elucidated. Here, we show that physiologically quiescent SM22α-lineage stromal cells expand after bone injury to regulate diverse processes of intramembranous bone regeneration. The majority of SM22α-lineage cells neither act as stem cells in vivo nor show their expression patterns. Dysfunction of SM22α-lineage niche cells induced by loss of platelet-derived growth factor receptor β (PDGFRβ) impairs bone repair. We further show that PDGFRβ-triggered hydrogen sulfide (H₂S) generation in SM22α-lineage niche cells facilitates osteogenesis and angiogenesis and suppresses overactive osteoclastogenesis. Collectively, these data demonstrate that non-stem SM22α-lineage niche cells support the niche for bone regeneration with a PDGFRβ/H₂S-dependent regulatory mechanism. Our findings provide further insight into non-stem bone stromal niche cell populations and niche-regulation strategy for bone repair.

Hedgehog Signaling Controls Bone Homeostasis by Regulating Osteogenic/Adipogenic Fate of Skeletal Stem/Progenitor Cells in Mice

Skeletal stem/progenitor cells (SSPCs) can differentiate into osteogenic or adipogenic lineage. The mechanism governing lineage allocation of SSPCs is still not completely understood. Hedgehog (Hh) signaling plays an essential role in specifying osteogenic fate of mesenchymal progenitors during embryogenesis. However, it is still unclear whether Hh signaling is required for lineage allocation of SSPCs in postnatal skeleton, and whether its dysregulation is related to age-related osteoporosis. Here, we demonstrated that Hh signaling was activated in metaphyseal SSPCs during osteogenic differentiation in the adult skeleton, and its activity decreased with aging. Inactivation of Hh signaling by genetic ablation of Smo, a key molecule in Hh signaling, in Osx-Cre-targeted SSPCs and hypertrophic chondrocytes led to decreased bone formation and increased bone marrow adiposity, two key pathological features of age-related osteoporosis. Moreover, we found that the bone-fat imbalance phenotype caused by Smo deletion mainly resulted from aberrant allocation of SSPCs toward adipogenic lineage at the expense of osteogenic differentiation, but not due to accelerated transdifferentiation of chondrocytes into adipocytes. Mechanistically, we found that Hh signaling regulated osteoblast versus adipocyte fate of SSPCs partly through upregulating Wnt signaling. Thus, our results indicate that Hh signaling regulates bone homeostasis and age-related osteoporosis by acting as a critical switch of cell fate decisions of Osx-Cre-targeted SSPCs in mice and suggest that Hh signaling may serve as a potential therapeutic target for the treatment of osteoporosis and other metabolic bone diseases. © 2021 American Society for Bone and Mineral Research (ASBMR).

Fundamentals of bone vasculature: Specialization, interactions and functions

Angiogenesis, hematopoiesis and osteogenesis are fundamental processes mediating complex and essential biological functions. In the bone marrow, endothelial cells (ECs) are a principal mediator of regulatory signals that govern hematopoietic and mesenchymal stem cells. EC and osteoblast interactions and niche functions of ECs are fundamental in maintaining bone health and coordinating repair and regeneration following injury. These cellular interactions are subject to dysregulation and deterioration under stress, aging, chronic disease states and malignancy. Thus, the prospect of manipulating the bone vasculature has tremendous potential to advance therapeutic interventions for the management of bone diseases. This review discusses the current state of vascular-skeletal tissue interactions focusing on osteoblast and hematopoietic stem cells interaction with ECs.

Protein tyrosine phosphatases in skeletal development and diseases

Skeletal development and homeostasis in mammals are modulated by finely coordinated processes of migration, proliferation, differentiation, and death of skeletogenic cells originating from the mesoderm and neural crest. Numerous molecular mechanisms are involved in these regulatory processes, one of which is protein posttranslational modifications, particularly protein tyrosine phosphorylation (PYP). PYP occurs mainly through the action of protein tyrosine kinases (PTKs), modifying protein enzymatic activity, changing its cellular localization, and aiding in the assembly or disassembly of protein signaling complexes. Under physiological conditions, PYP is balanced by the coordinated action of PTKs and protein tyrosine phosphatases (PTPs). Dysregulation of PYP can cause genetic, metabolic, developmental, and oncogenic skeletal diseases. Although PYP is a reversible biochemical process, in contrast to PTKs, little is known about how this equilibrium is modulated by PTPs in the skeletal system. Whole-genome sequencing has revealed a large and diverse superfamily of PTP genes (over 100 members) in humans, which can be further divided into cysteine (Cys)-, aspartic acid (Asp)-, and histidine (His)-based PTPs. Here, we review current knowledge about the functions and regulatory mechanisms of 28 PTPs involved in skeletal development and diseases; 27 of them belong to class I and II Cys-based PTPs, and the other is an Asp-based PTP. Recent progress in analyzing animal models that harbor various mutations in these PTPs and future research directions are also discussed. Our literature review indicates that PTPs are as crucial as PTKs in supporting skeletal development and homeostasis.

Mesenchymal stromal cell-derived septoclasts resorb cartilage during developmental ossification and fracture healing

Developmental osteogenesis, physiological bone remodelling and fracture healing require removal of matrix and cellular debris. Osteoclasts generated by the fusion of circulating monocytes degrade bone, whereas the identity of the cells responsible for cartilage resorption is a long-standing and controversial question. Here we show that matrix degradation and chondrocyte phagocytosis are mediated by fatty acid binding protein 5-expressing cells representing septoclasts, which have a mesenchymal origin and are not derived from haematopoietic cells. The Notch ligand Delta-like 4, provided by endothelial cells, is necessary for septoclast specification and developmental bone growth. Consistent with the termination of growth, septoclasts disappear in adult and ageing bone, but re-emerge in association with growing vessels during fracture healing. We propose that cartilage degradation is mediated by rare, specialized cells distinct from osteoclasts. Our findings have implications for fracture healing, which is frequently impaired in aging humans.

PDGFRα reporter activity identifies periosteal progenitor cells critical for bone formation and fracture repair

The outer coverings of the skeleton, which is also known as the periosteum, are arranged in concentric layers and act as a reservoir for tissue-specific bone progenitors. The cellular heterogeneity within this tissue depot is being increasingly recognized. Here, inducible PDGFRα reporter animals were found to mark a population of cells within the periosteum that act as a stem cell reservoir for periosteal appositional bone formation and fracture repair. During these processes, PDGFRα reporter⁺ progenitors give rise to Nestin⁺ periosteal cells before becoming osteoblasts and osteocytes. The diphtheria toxin-mediated ablation of PDGFRα reporter⁺ cells led to deficits in cortical bone formation during homeostasis and a diminutive hard callus during fracture repair. After ossicle transplantation, both mouse PDGFRα reporter⁺ periosteal cells and human Pdgfrα⁺ periosteal progenitors expand, ossify, and recruit marrow to a greater extent than their counterpart periosteal cells, whereas PDGFRα reporter⁻ periosteal cells exhibit a predisposition to chondrogenesis in vitro. Total RNA sequencing identified enrichment of the secreted factors Fermt3 and Ptpn6 within PDGFRα reporter⁺ periosteal cells, which partly underlie the osteoblastogenic features of this cell population.

Recent Advances in Single-Cell View of Mesenchymal Stem Cell in Osteogenesis

Osteoblasts continuously replenished by osteoblast progenitor cells form the basis of bone development, maintenance, and regeneration. Mesenchymal stem cells (MSCs) from various tissues can differentiate into the progenitor cell of osteogenic lineage and serve as the main source of osteoblasts. They also respond flexibly to regenerative and anabolic signals emitted by the surrounding microenvironment, thereby maintaining bone homeostasis and participating in bone remodeling. However, MSCs exhibit heterogeneity at multiple levels including different tissue sources and subpopulations which exhibit diversified gene expression and differentiation capacity, and surface markers used to predict cell differentiation potential remain to be further elucidated. The rapid advancement of lineage tracing methods and single-cell technology has made substantial progress in the characterization of osteogenic stem/progenitor cell populations in MSCs. Here, we reviewed the research progress of scRNA-seq technology in the identification of osteogenic markers and differentiation pathways, MSC-related new insights drawn from single-cell technology combined with experimental technology, and recent findings regarding the interaction between stem cell fate and niche in homeostasis and pathological process.

Isolation and in vitro characterization of murine young-adult long bone skeletal progenitors

Skeletal stem and progenitor cells (SSPCs) constitute a reservoir of bone-forming cells necessary for bone development, modeling and remodeling, as well as for fracture healing. Recent advances in tools to identify and isolate SSPCs have revealed that cells with multipotent properties are present not only in neonatal bone, but also in adult bone marrow and periosteum. The long bone metaphysis and endosteum have been proposed as an additional SSPC niche, although in vitro approaches to study their cellular and molecular characteristics are still limited. Here, we describe a comprehensive procedure to isolate and culture SSPCs derived from the metaphysis and endosteum of young-adult mice. Based on flow cytometry analysis of known SSPC markers, we found the presence of putative multipotent SSPCs, similar to neonatal bone tissue. In vitro, metaphyseal/endosteal SSPCs possess self-renewing capacity, and their multipotency is underscored by the ability to differentiate into the osteogenic and adipogenic lineage, while chondrogenic potential is limited. Expansion of metaphyseal/endosteal SSPCs under low oxygen conditions increases their proliferation capacity, while progenitor properties are maintained, likely reflecting their hypoxic niche in vivo. Collectively, we propose a validated isolation and culture protocol to study metaphyseal/endosteal SSPC biology in vitro.

Mechanically-regulated bone repair

Fracture healing is a complex, multistep process that is highly sensitive to mechanical signaling. To optimize repair, surgeons prescribe immediate weight-bearing as-tolerated within 24 hours after surgical fixation; however, this recommendation is based on anecdotal evidence and assessment of bulk healing outcomes (e.g., callus size, bone volume, etc.). Given challenges in accurately characterizing the mechanical environment and the ever-changing properties of the regenerate, the principles governing mechanical regulation of repair, including their cell and molecular basis, are not yet well defined. However, the use of mechanobiological rodent models, and their relatively large genetic toolbox, combined with recent advances in imaging approaches and single-cell analyses is improving our understanding of the bone microenvironment in response to loading. This review describes the identification and characterization of distinct cell populations involved in bone healing and highlights the most recent findings on mechanical regulation of bone homeostasis and repair with an emphasis on osteo-angio coupling. A discussion on aging and its impact on bone mechanoresponsiveness emphasizes the need for novel mechanotherapeutics that can re-sensitize skeletal stem and progenitor cells to physical rehabilitation protocols.

Type-I collagen produced by distinct fibroblast lineages reveals specific function during embryogenesis and Osteogenesis Imperfecta

Type I collagen (Col1) is the most abundant protein in mammals. Col1 contributes to 90% of the total organic component of bone matrix. However, the precise cellular origin and functional contribution of Col1 in embryogenesis and bone formation remain unknown. Single-cell RNA-sequencing analysis identifies Fap⁺ cells and Fsp1⁺ cells as the major contributors of Col1 in the bone. We generate transgenic mouse models to genetically delete Col1 in various cell lineages. Complete, whole-body Col1 deletion leads to failed gastrulation and early embryonic lethality. Specific Col1 deletion in Fap⁺ cells causes severe skeletal defects, with hemorrhage, edema, and prenatal lethality. Specific Col1 deletion in Fsp1⁺ cells results in Osteogenesis Imperfecta-like phenotypes in adult mice, with spontaneous fractures and compromised bone healing. This study demonstrates specific contributions of mesenchymal cell lineages to Col1 production in organogenesis, skeletal development, and bone formation/repair, with potential insights into cell-based therapy for patients with Osteogenesis Imperfecta.

Skeletal stem cell fate defects caused by Pdgfrb activating mutation

Autosomal dominant PDGFRβ gain-of-function mutations in mice and humans cause a spectrum of wasting and overgrowth disorders afflicting the skeleton and other connective tissues, but the cellular origin of these disorders remains unknown. We demonstrate that skeletal stem cells (SSCs) isolated from mice with a gain-of-function D849V point mutation in PDGFRβ exhibit colony formation defects that parallel the wasting or overgrowth phenotypes of the mice. Single-cell RNA transcriptomics with SSC-derived polyclonal colonies demonstrates alterations in osteogenic and chondrogenic precursors caused by PDGFRβD849V. Mutant cells undergo poor osteogenesis in vitro with increased expression of Sox9 and other chondrogenic markers. Mice with PDGFRβD849V exhibit osteopenia. Increased STAT5 phosphorylation and overexpression of Igf1 and Socs2 in PDGFRβD849V cells suggests that overgrowth in mice involves PDGFRβD849V activating the STAT5-IGF1 axis locally in the skeleton. Our study establishes that PDGFRβD849V causes osteopenic skeletal phenotypes that are associated with intrinsic changes in SSCs, promoting chondrogenesis over osteogenesis.